A bolometer is a device for measuring incident electromagnetic radiation. While bolometers can be used to measure radiation of any wavelength, they are often used to detect infrared and radio wavelengths, and more particularly sub-millimeter wavelengths (from around 200 μm to 1 mm wavelength). Bolometers are often used for astronomy at these wavelengths, but have applications in consumer electronics, such as cameras, and other products, such as automobiles.
In order to measure changes in resistance of a bolometer due to optical heating, an electrical current must be passed through the bolometer. In an array format, this is often done by sequentially pulse biasing bolometers or groups of bolometers. This causes heating generally significantly larger than the optical response. This heating causes two negative effects when attempting to operate the bolometer over a wide range of ambient temperature.
First, the resistance of the bolometer is changing significantly during the readout or pulse bias period. This makes low noise measurement significantly more difficult as the temperature and therefore the response of the bolometer is changing rapidly with time. Typical solutions have been to integrate an amplified detector response signal over the pulse bias period. Since the input signal to an integrator for a biased bolometer is a ramp, the integration results in a parabolic response as shown in FIG. 1. When pulse bias heating is significant as in the case of a longer pulse bias period or higher detector bias which is done to achieve more sensitivity, the integrated pulse bias response can cause a reduction in dynamic range due to the integrated potential changing significantly over the period, thereby using up allowable circuit potential swing or dynamic range. Compensation for the ramping reduction in resistance due to heating over the bias period is used to maintain a constant output over the pulse bias period such that dynamic range is not consumed by the changing detector resistance. This is particularly true as temperature is increased, reducing detector resistance and resulting in increased power dissipation and thereby increased ramping.
Second, the pulse bias heating causes a mismatch in average response when compared to a temperature coefficient of resistance (TCR)-matched reference resistor which is thermally connected to the substrate. A reference resistor is normally used to provide a temperature compensation matching bias to the active bolometer and thereby cancel much of the substrate temperature change effects. The circuit in FIG. 2 shows a series connection of an active thermally isolated bolometer with a substrate thermally connected resistor with the same TCR. Ideally the output voltage, Vout, would remain constant over temperature. However, due to the bias heating effects, the active bolometer resistance decreases faster (with increasing substrate temperature) than the reference resistor, since the bias heating increases due to the higher power dissipation with increased current, and therefore Vout decreases with increasing temperature. Additional compensation beyond using a simple matched reference resistor is required in order that Vout remains relatively constant over temperature such that an amplified detector signal remains well within the dynamic range of the readout circuitry if the bolometer is to be operated over a wide ambient temperature range.
Proposed methods for compensation of the output ramp voltage from a detector due to temperature rise during the pulse bias period are presented in Jansson, et al., “Theoretical analysis of pulse bias heating of resistance bolometer infrared detectors and effectiveness of bias compensation,” SPIE Vol. 2552, p. 644-652 (1995).
What is needed is an improved system and method for compensating for bias heating effects in a bolometer circuit when used in an array format. Further what is needed is a method of improving a bolometer array's output stability as a function of changing substrate temperature. Further what is needed is a system and method for reducing the change of the detector signals due to pulse bias heating from the bolometer circuit to allow the use of amplifiers and other components having a limited signal swing. Further what is needed is a system and method for improving the accuracy of the readout of bolometer detectors which are pulse biased, causing rapid shifts in detector temperature. Further still, what is needed is an improvement in stability which may enable a bolometer focal plane array to operate over a very wide range of substrate temperature, such as the typical commercial temperature range of −40 degrees C. to 85 degrees C., with little change in output signal compared to the dynamic range. Further still, what is needed is a bolometer focal plane which may operate without the need for a Thermo-Electric Cooler to stabilize temperature. Further still, what is needed is a bolometer focal plane which may operate without the need for controlling substrate temperature.
The teachings herein extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the above-mentioned needs.